R-SNARE ykt6 resides in membrane-associated protease-resistant protein particles and modulates cell cycle progression when over-expressed

Abstract

Background information: The arginine-type soluble N-ethylmaleimide-sensitive factor attachment protein receptor (R-SNARE) ykt6 possesses several atypical properties including selective high expression in neurons, a lipidated C-terminus, localization to punctae that do not correspond with known endomembrane markers, a potent ability to protect the secretory pathway from alpha-synuclein over-expression and specific up-regulation in tumors. We have followed up on several of these features that together suggest nontraditional SNARE structures and functions. Results: A significant portion of ykt6 in PC12 cells was found in a protease-resistant state suggestive of a large complex or aggregate. Other endoplasmic reticulum/Golgi SNAREs were not protease resistant, demonstrating that SNARE complexes per se did not cause protease resistance. Mutagenesis indicated that lipidation of the ykt6 C-terminus was also not involved, implicating its longin domain in particle formation. Immunogold electron microscopy revealed ykt6 labeling of ∼100nm electron densities associated with diverse membranes. Density gradient analysis of the protease-resistant structures confirmed their tight association with membranes. Since excess ykt6 has been correlated with tumorigenesis, we tested whether ykt6 over-expression in normal rat kidney cells that normally express little ykt6 affected the cell cycle. Ykt6 over-expression was found to result in altered cell division cycles as evidenced by significantly smaller cells, a higher mitotic index and increased DNA synthesis. Mutagenesis studies dis-correlated SNARE function with the cell cycle effects; instead, the cell cycle effects correlated better with ykt6 properties related to the longin domain or particle formation. Conclusions: The ykt6 particles/aggregates may represent ykt6 engaged in a non-SNARE function(s) or else nonfunctional, stored and/or excess ykt6. Whether the particulate ykt6 structures represent a means of buffering the apparent proliferative activity or are in fact mechanistically related to this activity will be of future interest in neuroscience and cancer biology. Ykt6 is an unconventional R-SNARE protein that resides in membrane-associated protease-resistant protein particles. Over-expression of ykt6 in cultured cells alters cell cycle progression. Surprisingly, neither the formation of protein particles nor the cell cycle effects require the C-terminal lipidation of ykt6 thought to be critical for SNARE function. The particulate structures and cell cycle effects may arise from novel features of the ykt6 longin domain. © 2012 Soçiété Francaise des Microscopies and Société de Biologie Cellulaire de France.